(1) Field of the Invention
The present invention relates to a laminated draw-formed container having excellent shock resistance (dent resistance) after the lapse of time and to a process for producing the same. More specifically, the invention relates to improving the shock resistance of a container which is produced by draw-forming a laminated material which comprises a metal plate on which a polyester film is laminated.
(2) Description of the Prior Art
Side-seamless cans have heretofore been produced by subjecting a metal blank such as an aluminum plate, a tin plate or a tin-free steel plate to at least one stage of the draw working between a drawing die and a punch thereby to form a cup which consists of a barrel without seam on the side surface thereof and a bottom that is connected to the barrel as a unitary structure in a seamless manner and then, as required, subjecting the barrel to the ironing between an ironing punch and a die thereby to reduce the thickness of the barrel. It has further been known to reduce the thickness of the side wall by bending and elongating the barrel at the curved corner portion of a redrawing die instead of ironing the barrel of the container Japanese Laid-Open Patent Publication No. 258822/1989.
Furthermore, in addition to a widely employed method of applying an organic paint onto the cans that have been formed, there has been known a method of coating a side-seamless can with an organic material by laminating a resin film in advance on the metal blank before it is being put to the forming. According to Japanese Patent Publication No. 34580/1984, a polyester film derived from terephthalic acid and tetramethylene glycol is laminated on a metal blank. In producing redraw-formed cans by bending and elongation, furthermore, there has further been known to use a metal plate coated with a vinyl organosol, a epoxy, phenolics, a polyester, an acryl or the like.
According to Japanese Laid-Open Patent Publication No. 101930/1991 filed by the present inventors, there has been described a coated metal plate for cans comprising a laminated material of a metal plate, a polyester film layer consisting chiefly of an ethylene terephthalate unit, and, as required, an adhesive primer layer that is interposed between the metal plate and the polyester film layer, wherein said polyester film layer has an X-ray diffraction intensity ratio as defined by the formula, EQU R.sub.X =I.sub.A /I.sub.B
wherein I.sub.A is an X-ray diffraction intensity by the diffraction plane having a plane gap of about 0.34 nm (CuK.alpha. X-ray diffraction angle is from 24.degree. to 28.degree.) which is in parallel with the surface of the polyester film, and I.sub.B is an X-ray diffraction intensity by the diffraction plane having a plane gap of about 0.39 nm (CuK.alpha. X-ray diffraction angle is from 21.5.degree. to 24.degree.) which is in parallel with the surface of the polyester film,
of from 0.5 to 15, and has an anisotropic index of the in-plane orientation of crystals of smaller than 30.
However, the metal blank which is coated with an organic material in advance is subject to be damaged by the tools during the step of draw-forming. The portions where the coating is damaged permit the metal to be exposed actually or latently; i.e., the metal elutes out or is corroded starting with these portions. In producing the seamless cans, furthermore, there takes place a plastic flow such that the size increases in the direction of height of the can and the size contracts in the circumferential direction of the can. In fact, however, the plastic flow deteriorates the adhesion between the metal surface and the organic coating. Moreover, residual distortion in the organic coating causes the adhesion between them to decrease with the lapse of time. Such a tendency appears particularly strikingly when the content is canned while it is hot or when the canned content is heat-sterilized at low to high temperatures.
With the draw-formed can made of the above-proposed laminate of a polyester consisting chiefly of a polybutylene terephthalate (PBT) unit, the film layer adheres well to the metal substrate when the degree of crystallinity thereof is suppressed to be small, and the laminated material can be worked fairly favorably. However, the film layer exhibits barrier property against the corrosive components which is about one-half that of the polyethylene terephthalate/isophthalate (PBT/I). There further arise problems in regard to sticking of the film to the tools and thermal resistance of the containers.
On the other hand, the latter draw-formed can obtained by using a laminated material on which the polyethylene terephthalate (PET) and PET/I films are laminated, exhibits excellent barrier property against corrosive components and can be worked fairly well. When a time passes after the content has been filled in the container that is worked, however, there arises a problem in that the coated film loses shock resistance and particularly loses dent resistance.
In this specification, the dent resistance stands for a test for examining whether the coating of the container withstands the breakage or the peeling when an impact or a shock is given to the container to such a degree that leaves trace of hit, and is a very important test for evaluating the practical durability of the cans.